The invention concerns a circuit for monitoring the state (open or closed) of an ac switch.
Switches are used for example in devices for controlling and monitoring the burner and the ignition device of oil-fired and gas-fired systems and for monitoring switches for control members such as fuel valves and air flaps, wherein a microprocessor evaluates the items of information supplied by way of mains voltage-carrying signalling lines and produces suitable control commands. Particularly in regard to the safety aspect which is required in the start-up procedure and in operation of oil and gas burners, the switch-off capability of the switch devices which switch loads which are critical in terms of safety (such as for example a fuel valve) has to be frequently checked in order to be able to detect a malfunction of the switch device before a danger situation arises.
German patent specification No 30 44 047 and German patent specification No 30 41 521 C2 disclose a control arrangement for oil burners, in which items of information about the switching states of relay and sensor contacts are transmitted to a microprocessor by means of amplifiers. The switching states of the relay contacts are supplied by way of mains voltage-carrying signalling lines to respective amplifiers each of which is connected on its output side to an input of the microprocessor so that the latter must have a number of inputs corresponding to the number of amplifiers. Separating members such as for example optocouplers or transformers are used for the galvanic separation of the signalling lines and the microprocessor. In that arrangement, there is one separating member per signal voltage. The microprocessor is programmed to implement a number of checking operations to ascertain whether a system with switched loads is actually passing through a switch-on phase in the correct fashion. For that purpose signals are read in by the microprocessor and compared to reference values. In the event of a defective load state, the microprocessor switches the loads off.
In addition, in an arrangement known from German laid-open application (DE-OS) No 41 37 204 for monitoring ac switches, mains voltage-carrying signalling lines are connected by way of optocouplers to an interrogation unit of an ac voltage detector. In that arrangement the signalling lines are connected to the optocouplers by way of respective low pass members each comprising a resistor and a capacitor connected in series therewith. The switching states of the ac switches are interrogated by way of the signalling lines, and stored. In an evaluation unit connected downstream of the interrogation unit, the switching states are compared to a reference statexe2x80x94open or closedxe2x80x94and in accordance therewith the switch state signal is formed, which contains at least one item of informationxe2x80x94error or no errorxe2x80x94overall for all ac switches involved. It is not possible to ascertain from the switch state signal which ac switch can no longer be switched off if need be, so that a simple display for diagnosis purposes is not possible.
European patents EP 660 043 and 660 044 also disclose circuits for monitoring ac switches. Those circuits however cannot be checked in a continuous mode in respect of the correct operability of their components which are relevant in terms of safety.
The object of the invention is to provide a circuit for monitoring an ac switch, with which a load can be switched to a mains voltage, which can be fed opposite the neutral point of the mains voltage and whose components which are relevant in terms of safety can be checked in a continuous mode of operation of the circuit at any time for correct operability thereof.
The invention provides a circuit for monitoring the statexe2x80x94open or closedxe2x80x94of an ac switch of which a first terminal is connected to a phase of a mains voltage, the circuit comprising:
an input which is connectable to a second terminal of the ac switch; and
first and second digital components;
wherein the input is connected by way of a diode and a first resistor to a high-ohmic input of the first digital component;
the high-ohmic input of the first digital component is connected by way of a second resistor to an output of the second digital component; and
in accordance with a predetermined time lapse, the output of the second digital component either is arranged to be connected to a neutral point of the mains voltage or carries a voltage which is greater than a threshold voltage of the input of the first digital component so that from the signal at an output, associated with the input, of the first digital component it is possible to deduce whether the ac switch is closed and the diode and the second resistor are intact or whether the ac switch is open.